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  • May 2017

    FORMLABS WHITE PAPER:

    Engineering Fit: Optimizing Design

    for Functional 3D Printed AssembliesTolerance and fit are essential concepts that engineers use to optimize the functionality of mechanical

    assemblies and the cost of production. Formlabs extensively studies the accuracy of our materials and

    works to maximize repeatability between prints and across printers. The data and best practices in this

    white paper can help Form 2 users to design functional assemblies that work as intended, with the least

    amount of post-processing or trial-and-error.

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 2

    Table of Contents

    Value of Tolerances in 3D Printing . . . . . . . . . . . 3

    Fit Selection. . . . . . . . . . . . . . . . . . . . . . . . 4

    Measuring and Applying Tolerance. . . . . . . . . . . 6

    Friction. . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    Lubrication . . . . . . . . . . . . . . . . . . . . . . . . 11

    Bonded Components. . . . . . . . . . . . . . . . . . . 11

    Machining Printed Parts . . . . . . . . . . . . . . . . .12

    Conclusion . . . . . . . . . . . . . . . . . . . . . . . .13

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 3

    Value of Tolerances in 3D PrintingIn traditional machining, tighter tolerances are exponentially related to increased

    cost. Tighter tolerances require additional and slower machining steps than wider

    tolerances. Machined parts are designed with the widest tolerances allowable for

    a given application.

    Unlike machining, where parts are progressively refined to tighter tolerances,

    stereolithography (SLA) has a single automated production phase. Proper

    dimensional tolerancing lowers post-processing time and ease of assembly, and

    reduces the material cost of iteration. Improper tolerances for a particular material

    can also result in broken parts, especially for press-fit components in brittle materials.

    Tolerance is the predicted range of possible

    dimensions for parts at the time of manufacture.

    Post-processing steps for printed assemblies include cleaning, sanding supports,

    and lubrication. Sanding an active surface is a reasonable method for achieving the

    correct fit if the part is a one-o!, because less tolerancing work is required in the

    design phase. With larger assemblies, or when producing multiples of something,

    proper dimensional tolerancing quickly becomes worthwhile.

    ACTIVE SURFACE Model region where two surfaces touch and either move

    against each other or have a static fit.

    With larger assemblies, or when producing

    multiples of something, proper dimensional

    tolerancing quickly becomes worthwhile.

    ShaftHoleBasic Size

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 4

    Fit SelectionThe functional needs of the assembly will define how parts should fit together.

    Free movement of a component requires clearance, or space between the active

    surfaces. This is achieved by ensuring that the tolerance zones of the active

    surfaces do not overlap. If no motion between parts is needed, a transition fit will

    allow for easy assembly and disassembly. A transition fit has partially overlapping

    tolerance zones.

    An interference fit provides a rigid, strong connection, but requires much more

    force applied in assembly.

    Note: Interference fit has fully overlapping tolerance zones and requires the use of

    a resin with greater elongation such as Durable, Tough, Flexible, or Standard.

    Small amounts of deviation in manufacturing means that engineering fit is a

    continuum rather than three completely separate stages. Larger clearance fits

    trade precision for freedom of movement. Tighter transition fits are stronger, but

    cause more wear on the connection. An interference fit that requires more force to

    join will be more challenging to disassemble.

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 5

    Fit can be divided into three categories–clearance, transition, and

    interference–and each category is defined by two types of fit.

    CLEARANCE A sliding fit will have some lateral play, while a running fit will have

    almost no play. A running fit has slightly more friction, but more

    accurate motion.

    TRANSITION With a keying fit, a component will accurately insert into or around

    another part, with only a light force needed to install and remove it.

    A push fit will require more force to join and remove the parts, but

    it will still be possible by hand.

    INTERFERENCE A force fit requires substantial force, likely with additional hand

    tools like a hammer to install, and is not intended to be removed.

    A press fit will need much more force applied by a tool such as

    an arbor press to install.

    PLAY The amount of space for movement in an unintended direction

    within a mechanism.

    Fit is a spectrum that can be divided into three categories, which help dial in fit based on the needs

    of your assembly. Color coding in this spectrum is applied to the graphs below.

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 6

    Measuring and Applying ToleranceFormlabs examined a variety of common geometries to find the real-life tolerance

    needs of each kind of fit. The two SLA materials tested were Tough and Durable

    Resin, which were designed to be used to print functional product prototypes

    that undergo mechanical and tribological (friction) stress. The following examples

    demonstrate why it is beneficial to apply the correct material to each application.

    Fit conditions in each graph are color coded: blue is a clearance fit, green is a

    transition fit, and orange is an interference fit.

    HOLE AND SHAFT

    A hole and shaft will usually require a clearance condition, which may range from

    a sliding fit to a running fit depending on needed accuracy. A running fit will require

    sufficient lubrication for free movement. The hole and shaft print is a useful general

    test of fit conditions for both Durable and Tough Resins.

    ALLOWANCE (MM)

    RE

    MO

    VA

    L F

    OR

    CE

    (N

    )

    -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

    100

    80

    60

    40

    20

    0

    Tough Results: 0.05 mm or greater of allowance resulted in

    a clearance fit between the hole and shaft. Between 0.0 mm

    and 0.0375 mm resulted in a transition fit—the shaft could

    be inserted with a normal amount of hand pressure. Below

    0.0 mm (an interference fit), the shaft becomes much harder

    to insert, and quickly exceeded the 55 N limit of the force

    meter. The shaft was able to be inserted with an arbor press at

    -0.0375 mm, but could not be removed.

    Durable Results: In this test, the Durable part transitioned from

    a transition fit to a freely insertable clearance fit at 0.0625 mm

    of allowance. At 0.0125 mm of allowance and below the parts

    had an interference fit and were difficult to remove. However,

    the parts could still be joined even at a negative clearance of

    -0.0375 mm. This is due to the high elongation of Durable Resin.

    Download Hole & Shaft Test Model

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 7

    BALL AND SOCKET

    A clearance condition should exist to allow the ball to rotate freely in the socket.

    However, there is a large interference between the radius of the ball and the

    socket opening. The socket opening needs to deform enough to be inserted, but

    not come out in normal use.

    Durable Results: In this test there was an inflection point between 0.2 mm and 0.3

    mm of allowance where the ball and socket becomes easier to separate. Between

    0.0 mm and 0.2 mm, the ball moves smoothly in the socket but stays in position. The

    interference fit in this range would be ideal for articulated figurines, which need to

    keep their orientation. Above 0.3 mm, the ball is slightly loose and moves freely in

    the socket. Below 0.0 mm, the ball could not be inserted at all.

    Tough Results: The ball and socket model was also printed in Tough Resin, but

    could not be inserted by hand because of the high friction and high strength of

    the material. Durable is better suited for a ball and socket geometry due to its

    wear resistance.

    Download Ball & Socket Test Model

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 8

    ROD AND BUSHING

    A bushing is a type of plain bearing designed for smooth, free movement along a

    rod. There should be a clearance fit between the rod and the bushing. Depending

    on the application, the clearance may be larger or smaller.

    Durable Results: Bushings printed in Durable Resin had smooth and low-friction

    movement over the polished steel guide rod. With a designed allowance of 0.025

    mm, the bushing showed a smooth running clearance fit with no play. At and above

    0.025 mm, the bushing did not grip the rod at all, allowing it to slide freely. Between

    0.0 mm and 0.025 mm, the bushing required a light force to push it over the rod.

    Below 0.0 mm of clearance an interference fit was observed that required significant

    force to slide the bushing over the rod, although the rod was insertable due to the

    compliance of the material. In this range the part isn’t functional as a bearing.

    Correctly lubricated, Durable Resin is a good choice for quickly prototyping custom

    bushings, gear systems, and smooth polypropylene-like products with moving parts.

    For kinetic assemblies that need to last for many motion cycles without wear, it is

    best to to use o!-the-shelf Delrin bushings embedded in Tough Resin parts.

    Note: Tough Resin is best suited for creating strong structural components that do

    not need to experience long term friction.

    Download Rod & Bushing Test Model

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 9

    Friction The amount of friction force between two components is the product of the

    force on the mating surface (directly related to fit), and a constant (the coefficient

    of friction), which is specific to each material. Coefficient of friction is useful for

    predicting how much resistance your parts will have to movement and wear, and

    how you can expect Formlabs resins to perform relative to other common materials.

    Formlabs tested coe!cient of friction using a weighted sled, a track, and a force meter.

    Below are the unlubricated coefficients of friction between the following pairs of

    materials printed with 100 micron settings:

    Parts were measured without any post-process smoothing or sanding. Durable

    Resin has the lowest sliding friction of Formlabs resins due to its high inherent

    lubricity. By contrast, Tough Resin has a higher sliding friction and markedly higher

    static friction which caused stick-slip behavior in our tests. Durable Resin’s lower

    coefficient of friction makes it best suited for moving components which interact

    in kinetic assemblies.

    Sliding components such as rails, pistons, and rods have lower friction if the contact

    surface area of the two mating surfaces is reduced. This is achieved by orienting

    objects in PreForm so that the layer “grain” pattern is perpendicular between the

    parts. If the grain is parallel the layer grooves will mesh, creating more surface area

    and higher static and kinetic friction.

    Coefficient of Static Friction (µs) Coefficient of Kinetic Friction (µ

    k)

    Durable | Durable 0.32 0.14

    Tough | Tough 0.77 0.2

    Durable | Tough 0.57 0.16

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 10

    Formlabs tested static and kinetic coefficient of friction between Durable components

    with parallel and perpendicular grain patterns by orienting the the sled 90° in Preform.

    In both the static and kinetic tests, the perpendicular orientation had lower frictional

    coefficients. The coefficient of static friction is impacted more significantly by

    grain orientation: in our tests, friction between perpendicular grain surfaces was

    43 percent lower than parallel grain surfaces. Kinetic friction was 11 percent lower

    between perpendicular grain surfaces.

    Friction between parts decreases over time as the surfaces experience wear. This

    is often beneficial to kinetic assemblies, and sanding and polishing is a deliberate

    example of wear. However, excessive wear tends to increase clearances between

    parts. Lubrication is the best way to reduce long term wear.

    Note: Durable Resin is designed to be the most wear-resistant Formlabs material,

    and is most suitable for for gears, bushings, and kinetic assemblies.

    In some cases—such as rollers, wheels, and robotic grippers—more friction is

    beneficial. For these applications, Flexible Resin has a higher coefficient of friction

    and lower lubricity.

    Most static and kinetic friction High static and moderate kinetic friction

    A micro-scale diagram of friction between surface orientations.

    Least static and kinetic friction

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 11

    LubricationLubricants are essential to keeping components running smoothly in kinetic

    assemblies. Mineral oil is an inexpensive and commonly available lubricant

    commonly used with SLA prints. Silicone oil based lubricants, such as Super Lube®,

    also work well, and last longer without becoming sticky.

    Bonded ComponentsIn order to bond printed components with adhesives, a narrow clearance fit is desired.

    Cyanoacrylate (Super Glue) will fill thin gaps due to its low viscosity. A syringe of resin

    cured by hand with a UV or blue-violet (405 nm) laser pen (and UV safety goggles) can

    be used to weld parts together in butt joints.

    Air-powered, functional scale model of a flat two-cylinder internal combustion engine printed in Durable

    and Tough Resin and lubricated with mineral oil.

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 12

    Machining Printed PartsThe most common post-processing steps for printed assemblies are sanding,

    polishing, and lubricating. Occasionally it can be useful to machine a plastic part

    after printing, for example, if the tolerances of a feature need to be tighter than 0.025

    mm, or to alter a feature after printing. Adding holes with a drill press or threads with

    a tap can be faster and more efficient than re-printing if the tools are available and

    the design has changed in the middle of a print.

    Tough and Durable Resin withstand machining the best of Formlabs range

    of materials due to their high strength and elongation. Other Formlabs Resins

    can also be machined, though they require more conservative techniques and

    faster tool speeds.

    Which machining operations work best with SLA resins?

    High Temp

    Sanding

    High-speed fly cutting

    High-speed, small-diameter boring and reaming

    Tough, Durable,

    Standard

    Sanding

    Face and peripheral milling

    Drilling

    Boring

    Reaming

    Tapping

    Flexible

    Sanding

    Drilling

    Cutting

  • FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 13

    ConclusionSpecifying fit based on material properties and mechanical function is a necessary

    practice in product engineering. The fit ranges shown for common geometries can be

    broadly applied to many designs to achieve functional prototypes with fewer iterations.

    For even more accuracy and an intuitive understanding of how parts will fit together,

    print the test models in a wider variety of materials and see how they perform.

    Beyond fit, choosing the right material is essential for creating working prints. Formlabs

    materials vary significantly in tensile strength, elongation, and wear resistance. Some

    basic geometries such as the rod and bushing will work significantly better in Durable

    Resin due to the high wear resistance and low friction of the material. Structural

    components under load would be most successful in Tough Resin, which has high

    tensile strength and a flexural modulus similar to ABS plastic.

    Download our Full Materials Data Sheet